1. Field of the Invention
The invention relates to a device, i.e. a unit for incorporation in a drill column for releasably connecting the drill column to a pipe or similar hollow member, comprising coupling members for attaching to the pipe and associated with piston-cylinder units having pistons adapted to be supplied at one end with a pressure medium for moving the coupling members to a different operating position and being spring-loaded at the other end for moving the coupling members back.
2. Description of the Prior Art
In drilling technology it may be necessary for a pipe or "casing" to be entrained downwards by a drill column, which usually comprises a number of parts and ends at the bottom in a drilling head or the like, without it being necessary or possible to use a special pipe-holder or releasing device. This happens inter alia when the drill column is relatively long and a casing, tubular shell, borehole lining or the like is needed only at the bottom of the drill column, i.e. mainly in boreholes under water in the offshore area, e.g. in the case of bored pipes and the like.
A unit comprising coupling members in the form of radially movable bolts can be incorporated in the drill column at the place which is intended to be near the top end of the entrained pipe or casing. The coupling members engage in holes in the pipe and can be held in position by hydraulic pressure. During drilling, they transmit rotation and axial motion from the drill column to the pipe. When the pipe has reached the desired depth during drilling, e.g. when it has reached hard ground after boring through an unstable area, the pipe has to be detached from the column. To this end, the retaining pressure on the bolts is relieved by a cable extending along the drill column from above ground, so that the bolts move radially inwards under spring pressure and come out of the openings in the pipe. The column can then be used for further drilling, whereas the pipe remains in position. A connection of this kind between a drill column and a pipe must be made above ground, since otherwise the bolts cannot go into the openings in the pipe. Also, it is impossible to couple the drill column to the pipe subsequently, as is desirable in certain cases.